1. Technical Field
The present invention relates to a positioning apparatus which uses radio waves from positioning satellites, a control method of the positioning apparatus, a control program for the positioning apparatus, and a computer readable recording medium for storing the control program for the positioning apparatus.
2. Related Art
Conventionally, positioning systems for positioning a current position of each GPS (Global Positioning System) receiver using a satellite navigation system such as GPS have been used.
Such GPS receiver receives a C/A (Clear and Acquisition or Coarse and Access) code, which is one of the pseudo random noise codes (hereinafter, referred to as PN (Pseudo random Noise code)) put on radio waves from GPS satellites (hereinafter, referred to as satellite radio waves), based on navigation messages indicating a GPS satellite orbit and the like (including rough satellite orbit information: almanac, and accurate orbit information: ephemeris). The C/A code is a code serving as a positioning standard.
The GPS receiver then identifies which GPS satellite transmits the C/A code, and calculates the distance (pseudo range) between the GPS satellite and the GPS receiver based on the time at which the C/A code is transmitted and the time at which it is received. The GPS receiver then positions the position of the GPS receiver based on the pseudo range of three or more of GPS satellites and the position of each GPS satellite on the satellite orbit (e.g., JP-A-10-339772).
Typically, coherent integration (hereinafter, referred to as coherent, simply) for correlating the received C/A code with a replica C/A code stored in the GPS receiver and integrating the correlation value, and incoherent integration (hereinafter, referred to as incoherent, simply) for integrating the result of coherent have been performed to increase the reception sensitivity (S/N ratio) of the C/A code.
Here, since both the aforementioned satellite orbit information and the C/A code are put on the satellite radio waves, the polarity of the C/A code with the bit rate of 1.023 Mbps may be reversed for every 20 msec by satellite orbit information of 50 bps. Therefore, there is a problem that the correlation values of the non-reversed C/A code and reversed C/A code compensate each other, resulting in decrease in correlation integration values obtained by the coherent processing, and thus reception sensitivity of the C/A code does not increase sufficiently.
On the other hand, a technology to increase the sensitivity by changing respective polarities of the PN signals (C/A codes) according to twenty types of pseudo patterns and synchronously adding the PN signals to changed polarities using a receiver terminal has been proposed (e.g., JP-A-2004-340855).
In addition, a technology to identify the polarities by multiplying information of navigation messages from base stations by the received C/A codes has also be proposed (e.g., U.S. Pat. No. 6,329,946B1).
However, according to the technology described in JP-A-2004-340855, only one of twenty pseudo pattern types can identify the polarity of the PN signals, it takes longer time for useless processing which is not used for positioning, and processing load of the GPS receiver increases because a large amount of data must be accumulated.
In addition, according to the technology described in U.S. Pat. No. 6,329,946B1, there is a problem that the accurate time of less than 1 msec is required for identifying a position where the navigation data reverses.